Spectral zonal information storage and retrieval

ABSTRACT

This disclosure depicts methods and means for implementing a novel optical information processing technique utilizing a phenomena (herein termed Fourier optical synthesis) involving effecting a complex amplitude addition of diffraction spectra characterizing two or more object functions. The processed object functions may represent totally different scenes, or color separation functions of a common colored scene. The disclosure stresses novel mosaic three zone spectral zonal encoding filters and methods of color information storage and retrieval using the novel filters.

United States Mueller 1? atent 1 [451 MarchG, 1973 t SPECTRAL ZONALINFORMATION STORAGE AND RETRIEVAL [75] Inventor: Peter F. Mueller,Concord, Mass.

[73] Assignee: Technical Operations, Incorporated,

\ Burlington, Mass.

[22] Filed: April 1, 1971 [21] AppLNo; 130,163

a Related U.S. Application Data [63] Continuation-impart of Ser. No.726,455, May 3, 1968, Pat. No. 3,664,248, which is a continuation-inpartof Ser. No. 564,340, July 1 1, 1966, abandoned.

[52] U.S. Cl. ..95/12.2, 178/54 CD, 355/40 [51] Int. Cl. ..G03b 33/00[58] Field of Search .......95/l2.2; 178/52 D, 5.4 CD; 355/32, 40, 77,71; 350/162 SF [56] References Cited UNITED STATES PATENTS 3,408,14310/1968 Mueller ..95/l2.2 X

1/1970 Smith ...355/32 X 3,488,190 3,533,340 10/1970 Macovski ..95/l2.23,585,286 6/1971 Macovski .350/162 SF Primary Examiner-"Robert P.Greiner Attorney-Rosen ls; Steinhilper [57] ABSTRACT This disclosuredepicts methods and means for implementing a novel optical informationprocessing technique utilizing a phenomena (herein termed Fourieroptical synthesis) involving effecting a complex amplitude addition ofdiffraction spectra characterizing two or more object functions. Theprocessed object functions may represent totally different scenes, orcolor separation functions of a common colored scene. The disclosurestresses novel mosaic three zone spectral zonal encoding filters andmethods of color information storage and retrieval using the novelfilters.

3 (Claims, 29 Drawing Figures I PAHZNIEBMR mm 3.719 1.27

sum 1m 6 WHBEQUELQ w 232 @Fifumm @Wum v. wmmsu FIG. 2 FIG. 3

INVENTOR BY= ALFREDHROSEN 0nd JOHN H COULT ATTORNEYS PE r51? [MUELLER'PATENTEDMR elm 3,719,127

7 SHEET 2 UF 6 PETEREMUELLEI? INVENTOR BY: ALFRED/F055 0 n JOHNHCOULTATTORNEYS PATENTED'HAR ems SHEET 3 OF 6 NEUTRAL RED FILTER CYAN FILTERRED FILTER SYNTEUESUS @WWAE 5CD WEBER R m. N L 5 U M 0 M 0 R E 2 T d NEM H AU. RV DONO F R ON A v B m F m nu E A 5 r H E WI U m N mw N M W SFIG. IO

PATENTEDMR 61973 SHEET l 0F 6- RaR GNG M f\ mm:

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FIG.

PETER FMUELLER INVENTOR BY= AL FREDH 0nd JOHN H ATTORN WAVELENGTH INMILLIMICRONS ROSE/V COULT EYS 3m wzbjmm PATENTED IIIIII '6 I975 SHEET 5[IF 6 50o WAVELENGTH IN MILLIMICRONS WAVELENGTH IN MILLIMICRONS FIG. 20

FIG. I9

WAVELENGTH IN MILLIMICRONS WAVELENGTH IN MILLIMICRONS FIG. 24

FIG. 22

C M C M C Y ND Y ND Y C M C M C ND Y ND Y ND Y I M c M c- M NDYNDYNDPETE/P E MUELLER INVENTOR BY= ALFRED H ROSE/V 0nd JOHN H COULT ATTORNEYSPmmnm arm SHEET 6 UF 6 FIG; 26

FIG. 25

FIG. 28

' FIG. 27

PETER F MUELLER lNVENTOR BY- ALFREO H. ROSE/V FIG. 29

and JOHN H. C ATTORNE SPECTRAL ZONAL INFORMATION STORAGE AND RETRIEVALCROSS-REFERENCE TO RELATED APPLICATION This application is acontinuation-in-part of application Ser. No. 726,455, filed May 3, 1968,now U.S. Pat. No. 3,664,248, which is a continuation-in-part ofapplication Ser. No. 564,340, filed'July 11, 1966 (now abandoned), andis related to application Ser'. No. 795,879 filed Feb. 3, 1969 (now U.S.Pat. No. 3,586,434), a continuation of application Ser. No. 564,340.

BACKGROUND OF THE INVENTION The invention disclosed and claimed in theabovereferenced application Ser. No. 726,455, now U.S. Pat. No.3,664,248, broadly concerns a technique for optically processinginformation which exploits a phenomenon (herein termed Fourier opticalsynthesis) in which spatial frequency spectra characterizing a pluralityof optically additive object functions which are respectively multipliedwith harmonically related carrier functions can be caused to add incomplex amplitude in a Fourier (frequency) space established in acoherent optical detection system.

This invention is concerned with one aspect of this broad inventiveconcept. More particularly, this invention concerns methods and meansfor implementing spectral zonal photostorage and retrieval according tothe said invention, especially by the use of novel mosaic three-zoneencoding filters. Spectral zonal filters according to this invention arecapable of encoding three spectral zonal images such that they may berecorded photographically, for example, and selectively retrieved by theuse of optical Fourier transform and frequency filtering techniques.

The filters for implementing this invention comprise a two-dimensionalpattern of mosaic filter units, each unit comprising four filterelements having different spectral characteristics. The filters, atfirst glance, appear to resemble certain of the mosaic screen platefilters fabricated circa the turn of the century to implement colorphotography by the so-called screen plate method. For example, see UJK.Pat. Nos. 23,812 (1907) Child and 6881 (1906) Smith.

It is important to understand, however, that the filters described inthe referent patents are far removed from the filters of this inventionin a number of important respects. First, they are used to recordelemental samples of color separation information for later directviewing through the same (or an identical) filter.

By integration of the light coming from a number of adjacent elements,the color value of the sum of a number of color elements is perceived.Stated another way, in the screen plate method of spectral zonalphotography, color information is both stored and restored at the recordon an element-by-element basis.

Second, the color separation information is not intended to be retrieved(and, in fact, is incapable of being retrieved) by the demodulation ofspatial carriers using optical Fourier transform and filteringtechniques, as is the case with information stored by the use of thefilters of this invention.

Further, the construction and spectral characteristics of these priorart filters is very different from that of the filters described andclaimed herein. The desirable spectral characteristics of screen platesis discussed in some detail in History of Three-Color Photography,American Photographic Pub. Co., (1925) by Wahl. As described in thisreference, it is desirable that screen plates have no neutral areas, andmandatory that they have no neutral areas with high transmissivity. Aswill become evident from the ensuing description and claims, onerequirement for the filters of this invention is that one of theelements of each mosaic unit have neutral spectral characteristics.

Another important distinction lies in the different geometry of thefilters the prior art screen plate filters in general comprise mosaicunits having three elements (usually red, blue, and green). The mosaicunits in the filters of the subject invention have four elements threehaving different spectral bandpass characteristics and the fourth beinga neutral element.

Perhaps more germane prior art with respect to the filters of thesubject invention are the striped spectral zonal filters which haveevolved during development of the art of diffraction photochromy. Forexample, see U.S. Pat. No. 2,813,146 Glenn (reissued as U.S. Pat. No.Re. 25,169); U.S. Pat. No. 3,378,633 Macovski; U.S. Pat. No. 3,378,634Macovski; and U.S. Pat. No. 470,310 Shashoua.

During the evolution of color television striped filters were developedwhich had the property that when multiplied with a scene image, colorsignals are impressed on spatial carriers of different frequencies.Scanning such a carrier-encoded image places distinct color informationon temporal carriers of different frequencies, thus allowing thediscrete color signals to be separated by frequency filtering in Fourierdomain. Examples of this approach are: U.S. Pat. Nos. 2,733,291 and2,736,762 Kell; U.S. Pat. No. 2,736,761 Sziklai; and U.S. Pat. Nos.3,291,901 and 3,300,580 Takagi. Another type of striped filter used inthe dot sequential method of color television was widely ex plored inthe late 1940s and 50s. An early example of this work is found in U.S.Pat. No. 2,452,293 De Forest.

Yet another color encoding filter for use in making half-tone screenplates is described and claimed in U.S. Pat. No. 3,085,878 Archer. Acursory inspection of each of these prior art patents will reveal thelack of pertinence thereof with respect to the novel mosaic filterdescribed therein.

OBJECTS OF THE INVENTION It is an object of this invention to providemethods and apparatus for recording and retrieving spectral zonalinformation.

It is another object to provide improved methods of diffraction processcolor storage and retrieval which is substantially free from cross-talklimitations imposed by prior art diffraction processes.

It is yet another object to provide improved spectral zonal encodingfilters for use in making records from which spectral zonal informationcan be retrieved in a coherent optical projector by Fouriertransformation and processing techniques.

Further objects and advantages of the invention will in part be obviousand will in part become apparent as the following description proceeds.The features of novelty which characterize the invention will be pointedout with particularity in the claims annexed to and forming a part ofthis specification.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding of theinvention reference may be had to the following detailed descriptiontaken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic illustration, grossly distorted for clarity, of acomposite record comprising interlaced record functions useful in thepractice of this invention;

FIGS. 2 and 3 show hypothetical objects useful in an illustration of theprinciples and practice of the invention;

FIG. 4 is a schematic exploded view of one step of a two-step contactprinting process which may be employed in the fabrication of a compositeoptical record useful in the practice of my invention;

FIG. 5 is a composite record comprising interlaced images of the objectsshown in FIGS. 2 and 3 which might be formed by the process illustratedin FIG. 4;

FIG. 6 illustrates, schematically and in exaggerated scale, a coherentoptical detection system for performing Fourier optical synthesis inaccordance with the invention;

FIG. 7 is a view of a composite optical record formed as a step in atechnique of two-color spectral zonal photography implementing myinvention;

F IG. 8 is a fragmentary schematic view of a spectral filter useful forforming the composite record shown in FIG. 7;

FIG. 9 shows a spatial filter mask useful in the practice of two-colorspectral zonal photography in accordance with this invention;

FIGS. 10-13 illustrate hypothetical objects useful in an illustration ofthe inventive concepts;

FIG. 14 shows a mask for assisting in interlacing on a common recordingmedium images of the objects in FIGS. 1013;

FIG. 15 depicts a composite record fabricated in the form of a mosaic,the mosaic comprising a plurality of mosaic units each having fourelements representing four distinct record functions;

FIG. 16 portrays a portion of the diffraction pattern which might beformed in a Fourier transform'space established within a coherentoptical system, such as shown in FIG. 6 of the FIG. 10 record;

FIG. 17 illustrates a spectral zonal filter implementing the principlesof this invention;

FIGS. 18-20 are diagrammatic representations of suitable spectralcharacteristics for blue, green, and red filter elements in the FIG. 17filter;

FIG. 21 depicts ideal spectral characteristics for the blue, green andred filter elements in the FIG. 17 filter;

FIG. 22 depicts ideal and suitable practicable spectral characteristicsof a neutral element comprising part of the FIG. 17 filter;

FIG. 23 is a highly schematic fragmentary representation of an opticalFourier transform of a record made using the FIG. 17 filter;

FIG. 24 is a schematic illustration of another filter which might beconstructed according to the principles of this invention;

FIG. 25 shows still another filter embodying this invention;

FIG. 26 depicts schematically a photostorage record as it might appearif exposed through the FIG. 25 filter to blue light and developed to apositive;

FIG. 27 depicts schematically an optical Fourier transform of a recordmade using the'FIG. 25 filter;

FIG. 28 schematically illustrates yet another embodiment of theinvention; and

FIG. 29 depicts schematically a photostorage record as it might appearif exposed through the FIG. 28 filter to blue light and developed to apositive.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The conceptual foundation ofthis invention, in a broad sense, as described and claimed in theabovereferenced copending application Ser. No. 726,455, involvesadditively combining a plurality of record functions respectivelymultiplied with harmonically related carrier functions, and, usingFourier transformation and spatial filtering techniques in a coherentoptical retrieval system, detecting selected functions representingcomplex amplitude additive (including subtractive) combinations of thespatial frequency spectra characterizing said record functions. Thecomplex addition of the record function spectra is accomplished byeffecting an optical interlacing of the record functions, either duringthe recording process '(e.g., by effecting formation of a compositerecord having the plurality of functions interlaced thereon), oralternatively, in a direct retrieval step (e.g., by effectivelyinterlacing in space the separate carrier modulating record functions).

As will become evident from the following description, an importantaspect of the described information processing concept lies in theestablishment of a predetermined spatial phase relationship between theoptically multiplexed record functions in order to achieve complexamplitude subtraction of the diffraction spectra produced by therespective record functions. In the interest of simplifying the ensuingdescription without intending a limitation on the scope of theunderlying principles, this phenomenon of complex amplitude addition ofthe diffraction spectra of different record functions is hereinaftertermed Fourier optical synthesis.

In order to further the understanding of the phenomenon of Fourieroptical synthesis, a mathematical analysis will be undertaken. Theprinciple is general and may be treated two-dimensionally. However, inthe interest of simplicity, the immediate analysis will be undertaken inone dimension only. Again, although the underlying mathematical andphysical concepts are completely general, the immediate description willbe in terms of a recording process involving the formation of acomposite optical record comprising two interlaced record functions.FIG. 1 depicts such a composite record 230. The record may be formed asfollows. A first record function representing an image intensitydistribution I (x,y) is multiplied by a one-dimensional periodic carrierfunction P(x) described as:

The amplitude transmittance of the record 230, after processing, can bedescribed as follows: ritx'iyenn'txln (x +K212(m (x) 1 (2) where K-v/2/E v/2 E, is the abcissa intercept and -y is the slope of theprocessing density-log exposure curve.

The Fourier transform of Eq. (2) is not, a) =ff fl (Kale, UT U K212,)-yI2P'( ))e21l(n;x+p y)d dy (3) and. .v

,then by the convolution theorem from ly =ff Fist-w, m w w TIKM'IQMII)va-i- Translated into physical terms, Equation 12) states i that aFourier transformation of the complex amplitude transmittance of therecord 230, processed to a photographic transparency comprises aconvolution of the spatial frequency spectrum of l,(x,y) with a Diracdelta function array of infinite components produced by carrier functionP(x) summed with a convolution of the spatial frequency spectrum of I(x,y) with a Dirac delta function array produced by carrier function P(x). it is important to note, for reasons which will become moreapparent below, that the spatial displacement between carrier functionsP(x) and P(x) has been transformed by operationof the Fourier integralinto a linear phase factor e appearing in the second term of Equation(12).

Considering only the spectra convolved about the delta functionsassociated with the (common) fundamental frequency (o"-=l/p) of carrierfunctions P(x) and P(x), i.e., the harmonic order n 1 (assuming in theinterest of simplicity, l and 1 to be frequency limited to 1/2p),

Thus, Equation (newsletter a complexamplitud'e T,.(u.v =1me "murmnses) 1.105

Thus, the coasts; enemas'asuntmrarniaasaa tional transform of thespectral difference function defined byequation (14) represents adifference between the images of Many) and I (x,y) formed independently.when 7 -2 (l have not found this to be a strict constraint) and t, tthere is generated the complex amplitude difference function T (u,v)lln'ke l I I,(u,v)- Mum) I lln'ke Alum) (l6) ii'iiiil may be recorded byconventional square law de tectors as the intensity distribution ertransform space containing a convolution of a spatial frequency spectrumassociated with each of the records with a Dirac delta function array.By the selection of carrier functions having one or more like harmoniccomponents and by effectively aligning the carrier functions (inherentlyachieved in an interlace simplified see-anthems antennas geometry) aspectral order associated with each of the record functions is caused tocoincide in transform space at least once. By establishing apredetermined displacement between the spatial phase of the carrierfunctions impressed on the record functions, there is caused a complexamplitude subtraction of the spectra of the first and second recordfunctions. Retransformation of the difference function thus formedproduces a two-dimensional display which represents the opticaldifference between the first and second record functions.

In a simple but dramatic application, my invention may be used to effectan optical subtraction of two totally different record functions.

Assume the functions to be synthesized comprise the words FOURIEROPTICAL as shown on record 232 in FIG. 2 and the words OPTICAL SYNTHESISas shown on record 233 in FIG. 3.

To prepare a composite optical record as described mathematically above,conventional photographic contact printing techniques may be used,although other methods are suitable. FIG. 4 is a schematic exploded viewof the contact printing method being applied, illustrating a portion ofthe record function 232 being contact printed through a grating mask 234to form an image on a photosensitive material 236 which represents amultiplication of the record 232 with the mask 234. The second record233 is interlaced with the first record 232 by replacing the record 232with record 233, shifting the grating mask 234 a distance equal toone-half the period p of the grating and exposing the photosensitivematerial 236 a second time. The composite record 238 thus formed wouldappear as shown in part in FIG. 5, the composite record functioncomprising the modulated words FOURIER OPTICAL being interlaced with thesecond record function comprising the words OPTICAL SYNTHESIS. Thus, thecomposite record 238 represents an additive combination of two recordfunctions respectively multiplied with spatial carriers having ahalf-period spatial phase displacement.

Various techniques may be employed for retrieving from the compositerecord 238 a function representing the difference in complex amplitudetransmittance hereinafter termed, in the interest of convenience, theoptical difference between the records 232 and 233. FIG. 6 schematicallyshows a system for effecting retrieval of the described opticaldifference function. The FIG. 6 system is illustrated as including lightsource means 240, comprising an arc lamp 242, lens 243, and aperturedmask 244, for generating an effective point source of high intensityluminous energy, a collimating lens 246, and a film gate 248 forsupporting an optical record 250. A transform lens 252 cooperating withthe collimating lens 246 forms an image of the effective point source ata plane termed the Fourier transform plane at which appears a Fraunhofer diffraction pattern of the record 250. A projection lens 254together with the transform lens 252 images the record 250 upon adisplay screen 256. The effective point source created by the lightsource means 244 and the collimating lens 246 produces opticalwavefronts having sufficient spatial coherence to produce a diffraction,pattern at the described Fourier transform plane which substantiallyrepresents a Fourier transformation of the complex amplitudedistribution across the record 250. In general, the diffraction patternof the record 250 represents a convolution of a spatial frequencyspectrum characterizing the record distribution with a Dirac deltafunction array produced by carriers on the record 250. In theillustrated example, the record functions are multiplied withazimuthally aligned carriers of like periodicity, and thus the Diracdelta function array produced by each of the record functions iscoincident in the Fourier transform space. However, by this invention,the spatial carriers respectively modulated by the record functions havea spatial phase displacement equal to one-half the fundamental carrierperiod p. Thus, the complex amplitude distributions produced by the tworecord functions will destructively interfere in the Fourier transformspace to produce a difference function representing a complex amplitudesubtraction of one record function from the other. This differencefunction may be selectively transmitted through the Fourier transformspace by placing a spatial filter mask 258 in the Fourier transformspace which has a pair of diametrically located apertures 260 thereinfor transmitting the fundamental (n l) diffraction orders produced bythe record 250. Thus, the display produced on the screen 256, comprisingthe words FOURIER SYNTHESIS represents the optical difference betweenthe record functions FOURIER OPTI- CAL and OPTICAL SYNTHESIS.

It is important to note that the complex amplitude addition (includingsubtraction) can be accomplished by forming the record functions to besynthesized in incoherent light and thus without the attendance of thenumerous limitations imposed by having to perform the recording step incoherent light as is required with holographic image synthesistechniques. It is also evident that the mode of operation of myinvention is substantially different from that of holographic techniquesin that, inter alia, the complex amplitude addition occurs in Fouriertransform space, rather than at the eventual output plane.

The illustrated photostorage and retrievalmethod and system is not to beinterpreted as being limiting in any sense. Numerous other techniquesare contemplated by this invention for achieving synthesis of opticalfunctions in accordance with the above-described principles. Forexample, there is no limitation on the practice of this invention to thesynthesis of binary images-as noted from the mathematical analysis, thenature of the record functions which may be synthesized is unrestricted.Continuous tone amplitude or phase images may be synthesized by mytechnique. The geometry of the carriers with which the record functionsto be synthesized are multiplied is again substantially withoutlimitation. For example, (as related to the described contact printingmethod) the transparent slits may be made much narrower than the opaquebars. Although a record thus formed would be inefficient in itsutilization of the film area, the operation of the principles of theinvention are not affected and complex amplitude addition would takeplace as described.

Alternatively, the transparent areas of the grating mask may in fact begreater than one-half the grating period. The result of the use of sucha grating geometry is that the interlaced record functions will overlapalong the elemental strip image margins. However, if the opticallyadditive relationship between the two record functions is maintained,the operation of the principles of the invention are not violated.Inorder to preserve this. additive relationship, the composite recordpreferably is linearly processed to a gamma of minus two in order thatthe complex amplitude transmission of the record is substantiallylinearly related to the intensity of the recording illumination. Thisrestraint is not restrictive; it has been found that considerablelatitude in processing may be tolerated without significantly affectingthe equality of the recovered images.

It is noted that optical subtraction may be achieved, as described,independent of the polarity of the processed composite record since itis a difference function, not an absolute function, which is sought.

A very significant application of the principles of my invention,described in part above, is in the field of spectral zonal photography.The production of true color reproductions of a colored photographicscene has engaged workers in the photographic arts since the beginningsof practical photography. One path along which studies were conductedhas led to the development of photosensitive materials capable ofphotostoring color information directly in all the hues of the scene.Another parallel path has been in the direction of storing colorinformation on panchromatic blackand-white film including techniques toretrieve the original color values from the colorless record. A verysubstantial effort some years ago was concentrated on the concept ofzonal recording of color information by imaging the photographic scenethrough a one (or two) dimensional mosaic spectral filter ontoblack-andwhite photostorage materials. See the abovereferenced U.l(.patents to Child and Smith. Retrieval of the color information from theblack-and-white record by this screen plate method requires exactregistration of the developed record with the taking filter to form atrue color reproduction of the scene. The registration and resolutionproblems inherent in this method have proven to be insurmountableobstacles to the commercial viability of this approach.

Yet another approach has involved the use of diffraction gratings tocolor code a black-and-white record. Such a technique is described inthe British Journal of Photography, Aug. 3, 1906, pages 609-612 byHerbert E. Ives; in a United States Patent to R. W. Wood, U.S. Pat. No.755,983, and in a U.S. Pat. No. to Carlo Bocca, 2,050,417. However, noneof these proponents of the use of gratings to color code information onblackand-white film succeeded in avoiding the need to make a pluralityof color separation records, and thus their attempts again encompassedthe registration limitation. Ives and Wood employ diffraction gratingsof disparate frequencies to enable the detection of particular colorinformation in a black-and-white record; however, such methods wereplagued by Moire interactions between the gratings. W. E. Glenn has alsoencountered these Moire beating effects in his exploration of the use ofdisparate frequency gratings in color systems, particularly inapplications to variable optical retardation systems utilizingdeformable thermoplastic recording media (see Vol. 48, No. 11, pp. 84l-3of the Journal of the Optical Society of America).

As suggested, techniques of Fourier optical synthesis may be utilized tophotostore and retrieve color imagery from a colorless recording mediumwithout many of the problems inherent in the above-described prior arttechniques.

I will explain below how by this invention Fourier optical synthesis maybe implemented in three-zone spectral zonal photography. However, in theinterest of simplicity in understanding the conceptual foundation andpractice of spectral zonal photography according to Fourieropticalsynthesis, I will first describe a two-color system of spectral zonalphotography utilizing but a single one-dimensional carrier functionduring the storage process.

A preferred way of implementing such a two-color system is to effect aninterlacing on a common blackand-white panchromatic recording medium oftwo record functions I ,,(x,y) and l (x,y), I ,(x,y) representing a cyancolor separation image of a colored photographic object and I (x,y)representing a full spectrum (herein termed for convenience white) imageof the same object. FIG. 7 illustrates, very schematically, how such acomposite record 298 might appear.

'The amplitude transmittance of record 298 processed to a transparencyis given by the relation:

If we consider only the spatial frequency spectrum at the n=0 order (00); Equation 19) reduces to:

swarm F [mama now.) 1 20) This distribution is essentially the cyanimage spectrum but not exactly since a red image contribution is. stillP ly}? Ma et ,P..tmm.-. Her/ever, Since Performing the retransformationof the spectral distribution defined by Equation (21) yields areconstruction, in coordinates u,v:

r( w( mAm (22) which represents a color separation which ispredominantly cyan in content.

' Considering now the fundamental (n=l) spectral order, o =l /p andEquation (19) reduces to 1 MD]- (2 But equation (24) can be reduced to140 2131 11) n 1= ra(#= /P I -v) which is the exact red separation scenespectrum.

Retransforming Equation (24) produces the relation:

A number of ways are available for implementing such a two-colortechnique. One way is to first record the full color scene in a normalcopy camera on a panchromatic black-and-white film through a subtractivefilter (cyan, for example), placing a grating over the exposed recordand re-exposing through a filter of the complementary color (red, inthis instance). Since the additive red and cyan exposures are equivalentto a full spectrum exposure, the resulting composite record representsan interlace of a full spectrum image with a cyan color separationimage. The record thus formed is preferably (although not mandatorily)reversal processed to a gamma of minus two, for example, by firstdeveloping for 5 minutes in DK-SO (2:1) at 68 Fahrenheit, washing for 30seconds, bleaching in a dichromate bleach for three minutes, washing andcleaning, flooding for seconds under a 100 watt lamp, developing asecond time in D-94 for 2 minutes, washing again, fixing,hyponeutralizing, and then drymg.

A preferred technique for producing such a composite record on which afull spectrum scene image is interlaced with a subtractive color (cyan,for example) separation image of the scene is to employ a spectralfilter in the nature of a grating having alternate neutral density andcyan filter strips, as shown fragmentarily at 299 in FIG. 8. Such afilter 299 may be fabricated in a number of ways, e.g., by colorphotographic techniques described in my above-referenced copendingapplication Ser. No. 795,879, now US. Pat. No. 3,586,434; or,preferably, by the use of multi-layer interference filter fabricationmethods.

With such a filter, a composite record as described is formed verysimply by erecting an image of the scene, multiplying the image with thefilter, and recording the multiplicative combination on a panchromaticblackand-white emulsion. In a preferred arrangement, the filter islocated at the plane of the first image formed of the scene in intimatecontact with the film.

After processing the exposed storage medium, a color reconstruction ofconsiderable fidelity may be retrieved from the record in a coherentoptical system substantially as shown in FIG. 6, described above, butmodified by the substitution of a spatial filter mask 300 as shown inFIG. 9 for the mask 258. The mask 300 has a pair of apertures 302 forpassing the first order spectra produced by the record and a thirdaperture 304 located on the optical axis for passing the zeroth orderinformation. Apertures 302 are covered by a red spectral filter andaperture 304 is covered by a cyan spectral filter in order that spectrapassed by mask 300 be transmitted in light having the correspondingwavelength characteristics. As described above, the informationtransmitted in the diffracted orders through the apertures 302characterize a spatial frequency spectrum associated with the redcontent of the photographed scene, and the information transmitted inthe DC. channel through the aperture 304 characterizes a spatialfrequency spectrum which represents predominantly the cyan content ofthe scene.

Records of the reproductions which I have generated using this system donot exhibit a full spectrum of natural colors, due to the inherentlimitations of two color systems and the described adulteration of thecyan spectra; however, the color reproductions are found to be veryaesthetically pleasing and highly saturated in the colors transmitted inpredominance by the system. Thus, a system has been described which forthe first time makes practicable spectral zonal photography withcolorless record media. The only additional requirement imposed by thedescribed system over conventional black-and-white photography is theintroduction of a spectral filter into the exposing light, as described.In its simplest application, a conventional camera is modified bypermanently locating a spectral filter, as described, at the image planeof the objective.

Thus far, the Fourier optical synthesis principles have beenmathematically and physically analyzed in terms of a one-dimensionalcarrier in the interest of simplicity. The underlying principles aremore general, however. Various difference signals can be generated byextending the basic concept of the one-dimensional interlace schemeabove to a two-dimensional scheme.

In the following analysis, let I,(x,y) represent the amplitudetransmittance of the i record image after processing. The totalamplitude transmittance of the record, then, is:

AM) 1( ,y) Po) o) 2w) P(x+p/2 Pry) Mm) +p/ (y+p/2) 4( v) (y+p/2)- TheFourier transform of:

E, is

where n represents delta function components (orders) in the u,dimension and m represents delta function components in the p.dimension.

Considering the orders n=:1, m 8 Up, 0)

Mer /p #m- (32) Considering the n=- "l m=:tl orders (8=l/p, =l/p),

Forn= l,m=il

Lur..-.1/p 1. And, finally for n=0, m=0

)+I4(l .r,My)]- (35) As indicated, the Equations (32-35) are general. In

one application of the invention, let

Mm) rony) Mm) 3( y)- (so Then the following reductions occur:

imam) in 1= l mas-Ham.) (31 AU zJ 'iIHn- 1=- l u imr lmur lp) (3s) Ill-1 AU xaI u) EU IMF P) 1/2 [when 120 respective objects, shifting themask 3118 after each exposure by one-half period p to expose the entirefilm area. A mosaic composite record is thus formed comprising atwo-dirnensional array of four element mosaic units, as shown in FIG.15.

Process the composite record to a transparency and locate same in acoherent optical system such as is shown in FIG. 6. The diffractionpattern formed, comprising a Fourier transformationof the complexamplitude transmittance function of the record, appears (in part) asshown in FIG. 16.

Assume that images 1,, l I and 1,, as shown in FIG. 15, are respectivelyimages of objects 1K, 1L, 11M, and 1N. Then, by selectively transmittingthrough Fourier transform space (with a spatial filter mask similar tomasks 258 described above) the n =il, m=0 orders, an image I (u,v) (theword FOURIER) alone is retrieved. Similarly, the words OPTICAL andSYNTHESIS alone may be recovered by filtering out all spectra in thetransform plane except the orders n=: -l, m==;t0; and n=0, m==il.Filtering for m =n =0 recovers the sum function I,,(u,v).

The results obtained from the assumption of E quation (36) areparticularly useful to implement a system for three zone spectralphotography. For spectral zonal photography all that is required is aparticularly simple mosaic filter 322 of the geometry shownschematically in FIG. 17 wherein the symbols G, R, B, and N.respectively represent green, red, blue, and neutral spectral filterelements.

The FIG. 17 filter will now be described in more detail. By way ofexample, the spectral characteristics of the red, blue, and greenelements of the FIG. 17 filter might be as shown diagrammatically inFIGS. 18, 19, and 20, respectively. The fourth element has beencharacterized above as having neutral spectral characteristics. If thespectral characteristics of the red, blue, and green filter elementswere ideal as shown, for example, in FlG. 21 at 324i, 326, and 328,respectively, the neutral element would be selected to have as nearly aspossible percent transmission throughout the visible region of theelectromagnetic wave spectrum. However, assuming the red, blue, andgreen filter elements to have practically achievable spectralcharacteristics as represented in FIGS. 118-20, then the neutral elementshould, ideally, have a spectral characteristic representing the sum ofthe spectral characteristics of the red, blue, and green elements, asrepresented by curve 33111 in P16. 22. Because of the practicaldifficulty in fabricating a neutral filter having the spectralcharacteristics depicted by the curve 330, in practice I have found thata neutral element having a spectrally flat transmittance characteristic,for example as represented by line 332 in FIG. 22, established at alevel representing substantially the average of the sum of the spectralcharacteristics of the red, blue, and green elements (i.e., the averageof curve 330), has yielded very satisfactory results.

Stated in another way, and considering for example the blue informationchannel, it is desirable (in order to avoid producing spuriousmodulation in a direction orthogonal to the direction of the blue signalcarrier) for the same amount of blue light (per unit of area) to passthroughthe neutral elements as passes through the blue elements. itfollows, then, that because practical spectral filters do not have aflat bandpass characteristic and reflect or absorb a certain amount ofthe light incident thereon, the neutral elements must be given anequivalent amount of average neutral density if this condition is to bemet. It can be readily envisioned from a study of FIG. 17 and thisspecification that if different amounts of blue light energy arerecorded on areas of a photosensitive medium located behind the neutraland blue elements of the filter, then upon development of the medium aspatial carrier will be formed on the record having a direction vectororthogonal to the direction vector of the blue channel. The directionvector of the blue channel is vertical in FIG. 17.

By similar reasoning, in order to prevent spurious modulation of bluelight by the red and green elements, the spectral characteristics ofthese elements should also be matched in their rejection of blue light.The above description of the blue channel applies equally to the greenand red channels.

As described above, to record spectral zonal information with filter322, the filter 322 is multiplied with an image of the scene to bephotographed and the product is recorded on a panchromatic emulsion.

Alternatively, the composite record may be made by four consecutiveexposures through a positionsequenced mask, such as mask 318 in FIG. 14,while appropriately imposing red, green, and blue spectral filters inthe exposure light path.

A record formed as described may be placed in a coherent projectionsystem similar to the system shown in FIG. 6. At a Fourier transformspace within the system (occupied by the mask 258 in FIG. 6) will appearan array of spectral orders surrounding the optical axis in a squaregeometry. The first spectral orders of such an array are shownschematically in FIG. 23. A mask similar to the mask 258 shown in FIG. 6but having apertures arrayed around the optical axis in the samegeometry as exhibited by the array of spectral orders shown in FIG. 23may be employed. Red, blue, and green filters would be located in themask at the positions noted in FIG. 23, whereupon a full colorreproduction of the scene recorded would be reproduced at the outputplane, which may be a screen as shown at 256 in FIG. 6.

Following the teachings of this invention, an alternative three-zonespectral zone filter may be fabricated which comprises a two-dimensionalsquare checkerboard-like distribution of mosaic units as in the FIG. 17filter, but which is made up of four elements, three of whichcharacterize optical subtractive primaries and thefourth of which hassubstantial neutral density. Such a filter is illustrated schematicallyin FIG. 24 wherein the letters C, M, Y, and ND. represent, respectively,cyan, magenta, yellow, and neutral density elements.

For reasons similar to those discussed above in describing the spectralcharacteristics of the FIG. 17 filter 322, it is desirable (consideringagain, for example, the blue information channel) that the same amountof blue light (per unit of area) which is transmitted through the yellowfilter is also transmitted through the neutral density filter. If itwere possible to fabricate a yellow filter having ideal spectralcharacteristics (i.e., 100 percent rejection of blue light in the blueregion, and percent transmission throughout the remainder of the visiblespectrum), then the ideal neutral density element would be infinitelydense and thus non-transmissive of any blue light. However, since suchideal filters are not practicable, in practice a certain amount of bluelight will pass through the yellow filter elements. It is desirable thatthe same amount of blue light transmitted through the neutral densityelements in order to assure that no spurious modulation is recorded in adirection orthogonal to the vectorial direction of the desiredmodulation (i.e., vertical in FIG. 24).

Again by reasoning similar to the above, it is desirable that the cyanand magenta filter elements pass equal amount of blue light (ideally 100percent).

The above description regarding the spectral characteristics of thefilter elements considering the blue channel applies equally to the redand green information channels.

In practice, I have found that rather than trying to match the rejectioncharacteristics of each of the yellow, cyan, and magenta elements to theneutral density element, very satisfactory results are obtained if theamount of neutral density in the neutral density element is balancedwith respect to the subtractive primary elements having in practice thebest rejection efficiency, normally the yellow filter elements. Anyerrors in failure to match the cyan and magenta elements with neutraldensity elements matched to the yellow filter elements will have nosignificant effect, I have found, on the fidelity of the reproductionsformed, since any errors in recording are significant only at very highexposure levels at which degradation of the reproduced images due tounrelated factors is more serious.

The product of a filter as shown at FIG. 24 and described above and animage of a colored object may be recorded on a panchromaticallysensitive black-andwhite emulsion as described above. placing adeveloped record taken through the FIG. 24 filter will produce in theFourier transform plane of a projection system as shown at FIG. 6 anarray of spectral orders identical in position and information contentto the array produced by the FIG. 17 filter, as shown at FIG. 23.

Whereas this invention has been described as being capable ofimplementation by mosaic filters having a generally checkerboard-likesquare geometry, implementation of this invention is not limited to sucha geometry. Other filters may be devised according to this inventionwhich comprise a two-dimensional pattern of like mosaic units arrangedso as to be periodic in two directions, each mosaic unit comprising fourfilter elements having different spectral exclusionary characteristics,the spectral characteristics of three of the four elements beingassociated primarily with different bands of radiation wavelengths, thefourth element being a substantially spectrally neutral element. Anexample is a filter having filter elements arranged in a hexagonalhoneycomb-like distribution such that each of the four elements issurrounded by three pairs of the remaining three elements, the pairconstituents of each pair located in diametric opposition.

Such a hexagonal filter is shown in FIG. 25 at 334. The filter 334 isillustrated as having spectral characteristics of the additive primarytype corresponding to the FIG. 17 checkerboard filter. The spectralcharacteristics of the red, blue, green, and neutral areas in thehexagonal filter 334 may be selected as described with respect to theFIG. 17 filter. It should be understood that the filter 334 and theother filters illustrated herein are greatly exaggerated in size, thefilter elements in a useful filter having, for example, -200 filterelements per millimeter. The filter 334 may be employed to recordspectral zonal information, as described above, for example, by placingit in contact with a photosensitive material disposed at the plane inwhich a scene image or reimage is formed.

FIG. 26 may assist in an understanding of the operation of the filter334 (and the other filter embodiments described above as well). FIG. 26is intended to represent diagrammatically a portion of a photographicrecord 336, developed to a positive, which has been exposed to a bluescene image, or to a blue portion of a multi-color scene image throughfilter 334.

The broken line literal notations designate the character of the lightwhich impinged on the record, as determined by the spectralcharacteristics of the filter 334. I

It can be seen from an inspection of FIG. 26 that the areas of therecord 336 which were located behind the blue and neutral filterelements received a high exposure since blue light passed through theblue and neutral filter elements substantially without attenuation. Theareas of the record 336 which were located behind the red and greenfilter elements are seen to have received a relatively low exposure(ideally zero), the green and red filter elements (assuming them to haveideal spectral characteristics) blocking substan- .tially all bluelight.

Thus, it can be seen that a spatial carrier for the blue sceneinformation is created, the spatial carrier (for the FIG. 25 filterorientation) having a horizontal direction vector. i

If a photographic transparency record, as shown at 336, is placed in. acoherent projection system as shown, for example, in FIG. 6, adiffraction pattern will be formed in a Fourier transform space withinthe system which represents an optical Fourier transform of thephotographic record. Such a pattern 338 is depicted schematically inFIG. 27, the first and zeroth spectral orders only being shown as theymight appear after appropriate color filtering as discussed below.

It can be seen from FIG. 27 that the pattern 338 has six first spectralorders representing three pairs of orders containing the blue, green,and red information arrayed around the optical axis. The first spectralorders carry literal notations designating the color information carriedby each. If red, blue, and green filters are placed in the correspondingfirst spectral orders, and the zeroth order (labeled W) is attenuated(or at least partially attenuated) a full color reconstruction of thescene will be reproduced at the output plane (occupied by the screen 256in the FIG. 6 system).

The FIG. 27 diffraction pattern illustrates progressively largerspectral orders at progressively greater distances from the optical axisfor information-bearing light of longer wavelength. This result followsfrom the well known laws of diffraction that longer wavelength light isdiffracted at a greater diffraction angle than light of shorterwavelength.

FIG. 28 illustrates yet another filter 340 having the same hexagonalgeometry as the FIG. 25 filter, but having spectral characteristics ofthe subtractive primary type as described above with respect to the FIG.24 filter. The spectral characteristics of the yellow, cyan, magenta,and neutral density elements in the FIG. 28 filter 340 may be selectedas described above with respect to the FIG. 24 filter elements.

FIG. 29 illustrates diagrammatically a photographic record 342,developed to a positive, which has been exposed through the FIG. 28filter. In order to show the functional equivalence of the FIGS. 25 and28 filters, FIG. 29 depicts the results of an exposure to a blue sceneimage, or a blue portion or blue content of a multi-color scene image.The exposed areas of the negative 342 are again labeled to designate thefilter elements through which each area was exposed.

It becomes manifest upon an inspection of FIG. 29, that the areas of therecord 342 which were located behind the cyan and magenta elements (eachof which transmit blue light without substantial attenuation) receive ahigh exposure and appear white (clear in a transparency) in the record342. Because blue light is substantially totally attenuated by theneutral density and yellow filter elements, the record 342 exhibits alow (ideally zero) exposure in the areas of the record 342 which werelocated behind these elements. Thus, again the blue scene information isrecorded as a modulation of a spatial carrier having (for the FIG. 28filter orientation and geometry) a horizontal direction vector. Therecord 342, if substituted for the record 336 in the FIG. 6 projectionsystem, would produce a substantially identical diffraction pattern, asshown in FIG. 27

It is significant to note in connection with a description of thehexagonal filters 334, 340, as contrasted for example with thecheckerboard filters illustrated in FIGS. 17 and 24, that theinformation is recorded with radial symmetry. All three color channelsare thus sampled at the same spatial frequency. Because the angularseparation between the playback channels (60) is greater than for theFIGS. 17 and 24 filters (45), larger spatial filter apertures may beemployed during playback and thus more light may be transmitted to theoutput plane.

Whereas storage of color information in three unique spectral zones bythe use of mosaic filters as described above has significant advantagesfor practical handheld photography, there are applications where thesame color information can be recorded by a sequential process, asdiscussed above with respect to FIGS. 114-15. Reference may be had for adescription of applicable multiple step sequential exposure processes tomy copending application Ser. No. 795,879, now U.S. Pat. No. 3,586,434.

The encoding filters described above may be fabricated by any of anumber of methods; however, employment of multi-layer interferencefilter fabrication methods are preferred because of the high maximumtransmission and the controlled spectral characteristics of the filterswhich can be produced by these methods.

The invention is not limited to the particular details of constructionof the embodiment depicted, and it is contemplated that various andother modifications and applications will occur to those skilled in theart. Filters having geometries other than those shown are contemplated.For example, the square checkerboard geometry of the FIGS. 17 and 24filters may be skewed to produce a mosaic filter in which the mosaicunits and individual filter elements 1 have a diamond shape. Further,the duty cycle of one or more of the filter elements (the fraction of acarrier period occupied by a filter element) may be varied to controlthe playback efficiency of one or more of the color channels.

Therefore, because certain changes may be made in the above-describedapparatus without departing from the true spirit and scope of theinvention herein involved, it is intended that the subject matter of theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is: 1. A method of optical information processing,comprising:

locating in coherent illuminating radiation a composite mosaic unitsrecord comprising four record functions respectively multiplied withfour twodimensional carrier functions of like fundamental periods butspatial phase displaced by substantially one-half of said fundamentalperiods in each of two period directions whereby each mosaic unitcomprises four elements respectively constituting portions of said fourrecord functions, said one record function representing a full spectrumimage of a colored scene and the remaining three record functionsrepresenting three primary color separation images; forming in a Fouriertransform space a diffraction pattern of said composite record includingfirst spectral orders containing different vectorial combinations ofcomplex amplitude distributions respectively associated with said fourrecord functions; and selectively transmitting through said transformspace at least one of said first spectral orders. 2. A method of makinga composite record for processing by Fourier transformation and spatialfiltering techniques, comprising:

exposing a photosensitive medium to form four record functionsrespectively multiplied with four two-dimensional carrier functions oflike fundamental periods but spatial phase displaced by substantiallyone-half of said fundamental periods in each of two period directions tothereby form a mosaic record, each mosaic of which comprises fourelements respectively constituting portions of four record functions,each of said four record functions representing a common colored scene,said method including forming said four record functions each in lightin a different frequency band. 3. A method of optical informationprocessing, comprising:

storing on a photostorage record, first, second and third spatialsignals respectively characterizing spectral zonal information in threedifferent bands of radiation wavelengths, and a fourth spatial signalcharacterizing substantially spectrally neutral information, multiplyingeach of said four spatial signals respectively with four two-dimensionalcarrier functions each having first and at least a portion of each ofsaid first spectral orders.

1. A method of optical information processing, comprising: locating incoherent illuminating radiation a composite mosaic units recordcomprising four record functions respectively multiplied with fourtwo-dimensional carrier functions of like fundamental periods butspatial phase displaced by substantially one-half of said fundamentalperiods in each of two period directions whereby each mosaic unitcomprises four elements respectively constituting portions of said fourrecord functions, said one record function representing a full spectrumimage of a colored scene and the remaining three record functionsrepresenting three primary color separation images; forming in a Fouriertransform space a diffraction pattern of said composite record includingfirst spectral orders containing different vectorial combinations ofcomplex amplitude distributions respectively associated with said fourrecord functions; and selectively transmitting through said transformspace at least one of said first spectral orders.
 1. A method of opticalinformation processing, comprising: locating in coherent illuminatingradiation a composite mosaic units record comprising four recordfunctions respectively multiplied with four two-dimensional carrierfunctions of like fundamental periods but spatial phase displaced bysubstantially one-half of said fundamental periods in each of two perioddirections whereby each mosaic unit comprises four elements respectivelyconstituting portions of said four record functions, said one recordfunction representing a full spectrum image of a colored scene and theremaining three record functions representing three primary colorseparation images; forming in a Fourier transform space a diffractionpattern of said composite record including first spectral orderscontaining different vectorial combinations of complex amplitudedistributions respectively associated with said four record functions;and selectively transmitting through said transform space at least oneof said first spectral orders.
 2. A method of making a composite recordfor processing by Fourier transformation and spatial filteringtechniques, comprising: exposing a photosensitive medium to form fourrecord functions respectively multiplied with four two-dimensionalcarrier functions of like fundamental periods but spatial phasedisplaced by substantially one-half of said fundamental periods in eachof two period directions to thereby form a mosaic record, each mosaic ofwhich comprises four elements respectively constituting portions of fourrecord functions, each of said four record functions representing acommon colored scene, said method including forming said four recordfunctions each in light in a different frequency band.